The internally pressurized hardwares such as tanks, boilers, valves, cylinders, pipelines, etc. require thermal safety against disastrous explosion. The thermal safety is generally provided by a fusible alloy which melts at an elevated temperature to allow the pressurized gas or liquid to be released. Therefore, said fusible alloy device is always under a high pressure and it is essential that said fusible alloy used as a fuse plug must withstand high pressure without any creep or extrusion or leakage. The zero creep requirement is also crucial for fusible links used in fire sprinkler systems as a fusible link for thermal safety, because said fusible link is under a constant load.
Fusible plugs for pressurized hardwares can be inserted/mounted to said tanks, cylinders, or such hardwares (plug insertion). Alternatively said hardware itself can have, for example, a threaded cavity and the fusible alloy can be filled into such cavity to seal said hardware (direct insertion). For a fusible link of fire sprinklers, alloy is filled into a cavity of a cylinder which acts as a link structure.
As the temperature and pressure conditions for creep strength become highly demanding, the intrinsic strength of the alloy, whether monolithic Or composite, reaches a limit and thus some kind of extrinsic design considerations must be given to share the applied load between the intrinsic strength of the alloy and extrinsic structure of the alloy-supporting member. This is especially so when the service temperature reaches an upper limit close to the melting point of fusible alloy. In general, alloys dispersed with reinforcing agents such as fibers, particles, shots, platelets, cubes, or any other geometrical materials i.e., composite alloys, can enhance the intrinsic strength of the matrix alloy but their intrinsic strengthening effect has a certain limit and beyond such limit the extrinsic strengthening effect must be generated to support a highly demanding load at an elevated temperature.
It is the aim of the present invention to provide such extrinsic strengthening effect via stress transfer or load sharing mechanism in which the applied normal load is shared both by the alloy-supporting or alloy-holding structural member and by the alloy itself. The load-sharing mechanism is achieved by a special design of the fuse plug geometry and also by the dense dispersion of reinforcing agents throughout the matrix alloy phase. The special design feature of said fuse plug consists of nonslip surface of the plug cavity containing the alloy in such a way that reinforcing agents are supported by the rough, rugged, nonslipping, threaded, and tapered surface, while such agents can be released away when the matrix alloy fuses to provides the mobility in case of fire, for instance.
When a fuse plug is under a high pressure, extrusional flow does not occur if the applied load is shared by the rigid PRD cavity structure in such a way that the stress born by the alloy is small enough not to break the intermetallic bonding force. If the normal extrusional stress is somehow transformed to shear stress, then the PRD wall structure can share the externally applied stress to alleviate the load born by the alloy. It is one of the functions of spherical shots to provide a spiral or zigzag path so that a shear force is exerted to shots and ultimately to the rigid wall via stress transfer mechanism.
The geometry of shots is generally spherical and the advantage of shots lies in increased compressive strength, uniform stress distribution, increased hardness, reduced stress cracking, excellent flow behavior, increased rigidity, dimensional stability, uniform dispersion, and high load capacity. Other geometries such as particles of irregular shape or fibers can be used as reinforcement but the flow behavior deteriorates with the increase of their content,
Another known problem is the stress corrosion of brass, the source of stress being the torque in threaded regions. Because of this stress corrosion cracking, the PRD must be replaced with the new one quite often to shorten the service life of PRD. Such stress corrosion effect can be solved by increasing the thickness of threaded part, by using the body material more resistant to corrosion such as phosphor bronze, or by coating the conventional brass with corrosion-resistant material such as tin, nickel, chromium, or other inert films.
The length of the cavity must be greater than a minimum to meet the required creep resistance. If the cavity length/diameter ratio is less than a certain minimum value, the creep strength is not met even though all other conditions such as nonslip PRD internal surface and strong reinforcement of the matrix alloy are satisfied.
It is the aim of the present invention to outline the detailed conditions for creep strength in an integrated fashion.